The present invention relates generally to magnetic resonance (MR) imaging and, more particularly, to reducing vibration-related artifacts in diffusion weighted MR imaging.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, or “longitudinal magnetization”, MZ, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1 is terminated and this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx, Gy, and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
As described above, magnetic field gradients are applied in MR imaging to encode spins in an object. It is desirable that the spatial frequency coordinates, i.e., k-space coordinates kx, ky in cycles/cm, of raw data samples acquired during MR imaging be controlled solely via the integrals of the applied encoding gradients. Ideally, these gradients impart directional and linear phase dispersion to the spins inside the object. The raw data's peak occurs at the point where minimal phase dispersion occurs across the object, i.e., when the integrals of the applied gradients go to zero. The gradient waveforms and analog-to-digital conversion are typically synchronized such that the raw data peak location is known with respect to the raw data matrix dimensions.
However, a number of undesirable mechanisms can cause shifts where the peak occurs within the raw data matrix. For example, gradient amplifier infidelity, gradient pulse induced eddy current fields, gradient channel group delays, and pulse sequence database timing errors can cause such shifts. Another undesirable mechanism causes residual phase dispersion across the object where object motion occurs during the application of the gradients. When undesirable mechanisms motion occurs, the phase “unwinding” imparted by a later gradient pulse does not align spatially inside the object with the previous phase “wind up” imparted by an earlier gradient pulse.
The subsequent shift in the raw data peak can be problematic in MR imaging methods where the raw data sampling is stopped or truncated near to the ideal assumed position of the peak and, in particular, diffusion weighted sequences. For example, “fractional ky” Diffusion Weighted Echo Planar Imaging (DWEPI) may be affected where the ky sampling may be terminated as close as 16 rows below ky=0. Another diffusion weighted technique is Diffusion Weighted Periodically Rotated Overlapping Parallel Line with Enhanced Reconstruction (DW-PROPELLER), and it can be particularly sensitive to k-space center shifts because the blade height, i.e., minor axis direction, is typically 8, leaving only 4 samples above and below the ideal peak. In this manner, the k-space peak does not have to shift far from the blade center along the minor axis before significant low spatial frequency data is lost, i.e., never taken.
The MR imaging methods of DWEPI and DW-PROPELLER typically use two high amplitude, long diffusion sensitizing gradient pulses to attenuate the signal from tissue diffusing along the applied DW gradient. These pulses make the DW sequences particularly sensitive to object motion induced k-space center shifts since the first diffusion pulse imparts a large phase shift (cycles/cm) across the object before the second one “removes” the phase shift, all under the assumption that the object has not moved during the diffusion pulse pair.
The direction and extent of object motion with respect to the applied diffusion gradient direction affects the extent of resulting k-space shifts. Rotational object motion about an axis “A” during application of diffusion pulses on another perpendicular axis “B” can result in an undesirable linear phase dispersion error along axis “B” and the remaining orthogonal axis “C”. If axis “B” and/or “C” happen to be image plane encoding axes, i.e., phase or frequency, k-space center shifts can occur. Rotational motion occurring about a physical z (object S/I) axis with DW gradients on physical X (object R/L) axis is problematic for DW-PROPELLER imaging, and shifts along Kminor (ky) are problematic for DWEPI. Object motion can be object derived voluntary/involuntary motion and/or scanner driven involuntary object motion caused by movement or vibration of mechanical components supporting the object such as a head coil, table, etc. Gradient pulse induced forces exerted on the gradient coil assembly can lead to involuntary object motion, which can cause artifacts in DWEPI and DW-PROPELLER imaging.
It would therefore be desirable to have a method and apparatus capable of reducing motion artifacts caused by scanner-induced involuntary object motion.